The present invention relates to engine cradle assemblies for motor vehicles and a method for forming the same.
Typical motor vehicle frames are constructed as a ladder frame assembly or as a uni-body type. For a description of a recent advancement in ladder frame assemblies, see U.S. Pat. No. 5,561,902, hereby incorporated by reference. The present invention relates to an engine cradle assembly, which is to be secured to a forward portion of a uni-body vehicle frame. The engine cradle is typically provided beneath the vehicle engine and is adapted to have the engine mounted thereon. The engine cradle is also adapted to mount the vehicle front end suspension, steering hydraulics and any other vehicle components deemed appropriate.
The cradle assembly must be of a sturdy construction as it is exposed to large mechanical stresses due to the weight of the engine and the entire weight of the vehicle (and transport load) transferred through the cradle assembly to the front end suspension. The cradle assembly is typically made from a pair of spaced side frame members having front and rear ends, a front cross frame member connected across the front ends of the side frame members, and a rear cross frame member extending between the rear ends of the side frame members. A suspension mount structure is typically mounted on the spaced side frame members and is adapted to mount the vehicle front end suspension. Because of the aforementioned mechanical stresses, especially at the portion of the cradle assembly at which the vehicle suspension is to be mounted, it is desirable to manufacture the cradle assembly with particular "strong points" to accommodate for such large localized stresses.
While it is important to manufacture the cradle assembly with sufficient strength, it is equally important that the cradle assembly not be made from excessive amounts of metal, otherwise material costs and the vehicle weight will be unnecessarily high. Thus, for the most part, the frame members of the cradle assembly are hollow (tubular) in form.
The patent literature discloses that one way to manufacture tubular vehicle frame members is to utilize a hydroforming technique. For example, U.S. Pat. No. 5,339,667 discloses a hydroforming method in which a round tubular blank is placed in a hydroforming mold and then radially expanded by about 5% to form a frame member with a generally box-shaped cross sectional configuration. The process disclosed in this patent forms a completed frame member that has its wall thickness uniformly thinned and not strengthened at any particular location by increasing the amount of metal per unit length thereat. A hydroforming technique is also discussed in U.S. Pat. No. 5,333,775, which discloses a method of manufacturing certain portions of a hydroformed member stronger than others by providing plural tubular blank portions of different wall thicknesses welded end-to-end, so that the completed hydroformed member will have a greater wall thickness at desired locations. The method disclosed in this patent is rather tedious and does not produce hydroformed frame member by modifying a tubular blank during hydroforming process in which the amount of metal per unit length is varied so as to strengthen certain portions of the completed frame member relative to others. Moreover, none of the related art discloses the use of hydroforming to form a engine cradle assembly or that such an assembly should be strengthened by providing relatively more metal per unit length at a position which is to have a suspension mount structure secured thereat so as to accommodate the localized stresses thereat.
There is thus a need to manufacture a cradle assembly which overcomes the limitations noted above. It is therefore an object of the present invention to provide a cradle assembly which satisfies this need. In accordance with the principles of the present invention, there is provided a cradle assembly for a motor vehicle comprising a pair of spaced side frame members, a front cross frame member and a rear cross frame member. The side frame members have front and rear ends, and the front cross frame member is connected with the front ends of the side frame members. The rear cross frame member extends between the rear ends of the side frame members. The frame members are formed from tubular metal blanks by a hydroforming process in which each blank has fluid pressure provided internally thereto and thereby radially expanded so that the frame members have a peripheral configuration modified with respect to an original peripheral configuration of the associated blanks. The side frame members have the amount of metal per unit length varied during the hydroforming thereof so as to present at least one longitudinal area at which i) a transverse cross-section defined by the modified peripheral configuration is increased by more than 5% of an original cross-section and ii) the amount of metal per unit length is greater than at least some other areas of the side frame members. The side frame members each have a rigid suspension mount structure constructed and arranged to mount a vehicle suspension member to the associated side frame member thereof. The rigid suspension mount is rigidly secured to the associated side frame member at the at least one longitudinal area having the transverse cross-section defined by the modified peripheral configuration increased by more than 5% of the original cross-section.
It is a further object of the present invention to provide a method for manufacturing a cradle assembly in accordance with the principles of the present invention. The method according to the present invention comprises forming a pair of side frame members, a front cross-frame member and a rear cross-frame member. The frame members are formed from associated metallic tubular blanks in a hydroforming process in which each blank is formed by placing the metallic tubular blank into a cavity of a die mold, the die mold having an interior surface defining a shape of the cavity, and providing a fluid internally to the metallic tubular blank with sufficient pressure so as to expand the blank outwardly so that an exterior surface thereof is moved into peripheral surface engagement with the interior surface of the die mold to substantially conform the blank to the shape of the cavity, and so that the frame members have a peripheral configuration modified with respect to an original peripheral configuration of the associated tubular metal blanks. The side frame members have the amount of metal per unit length varied during the hydroforming thereof so as to present at least one longitudinal area at which i) a transverse cross-section defined by the modified peripheral configuration is increased by more than 5% of an original cross-section, and ii) the amount of metal per unit length is greater than that of the remainder of the side frame members. The method further comprises securing a suspension mount to each side frame member at the at least one longitudinal area thereof having the transverse cross-section defined by the modified peripheral configuration increased by more than 5% of the original cross-section; and connecting the front cross-frame member with forward ends of the side frame members, and the rear cross-frame member with rearward ends of the side frame members.
The present invention also relates to the connection between frame members, for example, the connection between the rear cross frame member and the respective rear ends of the side frame members in the instance in which the rear cross frame member and side frame members are not provided as an integrally formed assembly. Such a connection is typically accomplished with particular frame connectors which prohibit the exterior surfaces of the frame portions which are to be connected from establishing surface-to-surface contact. More particularly, each frame member is typically defined by a substantially rectangular transverse cross-section defined by two pairs of spaced opposite walls. Each of the walls includes an interior and exterior surface, and the frame member has fastener receptacles which enable it to be fastened with another frame member. The fastener receptacle is defined by aligned openings in the spaced opposite walls of at least one of the pairs of walls and a sleeve member extending through the aligned openings. The sleeve member serves as reinforcement for the aligned openings and has opposite ends thereof extending radially outwardly to define opposite flange portions engaging the exterior surface of each of the opposite walls. It can be appreciated that when another frame member is positioned to have the fastener receptacle thereof aligned with the fastener receptacle of the first frame member for securement therebetween with a fastener, flange portions of each receptacle are brought into engagement. This engagement creates a slight gap between the exterior facing surfaces of the flange members to be secured at the areas to be joined. This results in an inherent weakness in the joint, and thus in the entire cradle assembly or vehicle frame incorporating such type of connections.
It is thus a further object of the present invention to provide a cradle assembly which overcomes the aforementioned problem. In accordance with the principles of the present invention, there is provided a cradle assembly for a motor vehicle comprising a pair of spaced side frame members having front and rear ends, a front cross frame member connected with the front ends of the side frame members, and a rear cross frame member extending between the rear ends of the side frame members. The rear cross frame member and the side frame members each have a substantially rectangular transverse cross-section defined by a pair of spaced horizontal walls and a pair of spaced vertical walls, each of the walls including an interior and exterior surface. The rear cross frame member have fastener receptacles extending through the spaced horizontal walls thereof at opposite ends thereof, and the side frame members have fastener receptacles extending through the spaced horizontal walls thereof at the rear ends thereof. The fastener receptacles at the opposite ends of the rear cross frame member are aligned with respective fastener receptacles at the rear ends of the side frame members. Fasteners each extend through the respective aligned fastener receptacles at the opposite ends of the rear cross frame member and the rear ends of the side frame members. The fasteners fasten each of the opposite ends of the rear cross frame member with the respective rear ends of the side frame members. The rear cross frame member have the exterior surface of one the horizontal walls thereof disposed in surface-to-surface engagement at each opposite end thereof with the exterior surface of the respective rear end of one of the side frame members.
It can be appreciated that the development of the particular frame structure which permits this surface-to-surface engagement is also an important object of the present invention. It is thus an object of the present invention to provide a frame structure for use in a motor vehicle frame structure, such as a cradle assembly. The frame structure comprises an elongate frame member having a substantially rectangular transverse cross-section defined by two pairs of spaced opposite walls. Each of the walls includes an interior and exterior surface. The frame member has aligned openings in the spaced opposite walls of at least one of the pairs of walls. In addition, a sleeve member extends between the aligned openings and has one end thereof annularly welded to the interior surface of one of the walls within the at least one pair about the opening therein. The sleeve member includes an intermediate portion extending from the one end through the interior of the frame member and outwardly from the interior of the frame member through the aligned opening in the other of the walls within the at least one pair. The sleeve member has an opposite end extending radially outwardly so as to define a flange portion engaging the exterior surface of the other of the walls about said aligned opening therein.
The principles of the present invention will best be understood by reference to the following description and the appended claims.